For use in small portable televisions or laptop computer displays, both monochrome and color liquid crystal displays (LCDs) have become popular. LCD panels have many significant advantages over light emitting diode (LED) displays and cathode ray tubes (CRTs). For example, because LCD panels have very low power dissipation, they are useful for battery-operated equipment. Moveover, because of LCDs' high ambient light levels, they are in utilized in equipment for use outdoors. Also, for displays that require custom shapes and symbols or displays with many digits or characters, LCD's high pixel density is preferred over the low pixel density of LEDs. Because LCDs are compact in size, energy efficient, and have the potential 1or high performance, LCDs are considered very promising.
A significant limitation of twisted nematic LCD displays is the relatively long switching time and resultant limitation on driver multiplexing due to the physical nature of the LCD phenomenon. Therefore, although acceptable for displays with modest information content and interactivity, available twisted nematic LCDs are not yet competitive with CRTs requiring high information content and interactivity. To overcome the multiplexing problems, a transistor driver can be provided for each pixel, however, this solution is accompanied by a substantial increase in display complexity and cost.
Color LCDs are available, however, because they suffer even more than their monochrome counterparts from complexity and slow switching time, the technology is not competitive with color CRTs except where low power and thin-form factor is critical.
Super twisted nematic (STN) LCDs have become popular in many applications because their multiplexibility is superior to other types of liquid crystals. STN cells exhibit bi-stable behavior when they switch rapidly from a deselect state to a select state and back again as the excitation root mean square (RMS) voltage crosses a switch threshold. The select and non-select regions can be made quite close to one another and permit cells to be multiplexed at high rates. The use of STN cells with their superior multiplexibility avoids panel complexity because each cell does not need its own transistor driver.
While a benefit of STN cells is their high multiplexibility, their detriment is that they are optically anisotropic crystals having two indices of refraction and therefore they exhibit undesirable birefringence effects. Their double refractance makes them unsuitable, particularly for black and white LCD panel applications because the STN cell unavoidably produces colors. An STN cell polarizes light of different wavelengths differently during their passage through the cell, and as a result an STN cell cannot be operated in black or white with a high contrast ratio. Moreover, the colors produced by birefringence are regarded as too limited in range and too inferior in quality to be suitable for use in color displays.
The birefringence operating mode of STN cells, however, has been exploited by different arrangements. For example, since the degree of the birefringence is a function of the voltage applied to the liquid crystal material, by switching the applied voltage to different values, different colors can be produced by a properly configuring of the STN cells. For example, by utilizing this inherent birefringence property, an arrangement is shown in U.S. Pat. No. 4,917,465 where STN cells are stacked to form a plurality of tri-pixels tuned to generate different subtractive primary colors (i.e. yellow, cyan, and magenta).
In a modification of the tri-subpixel stacked LCD configuration, U.S. Pat. No. 4,966,441 shows a bi-subpixel system which allows the second panel to have twice as many pixels as the first. In the bi-subpixel arrangements, the colors are generated by both birefringent color and color filters. In principle, all eight basic colors can be generated with these approaches, however, because black and white transmission is poor, they both suffer from low contrast ratios. In an optimized configuration of that shown in U.S. Pat. No. 4,966,441, only about 25% of the light transmits through the cell at the select state and when the cell is in the non-select state, and is therefore turned off, about 5% of the light still passes through the cell, therefore reducing its transmissivity characteristics.
Another common approach to achieve color display with STN panel uses adjacent tri-pixels covered with mosaic color filters. Tri-subpixels are usually placed in a "parallel" or side-by-side way as compared to the stacked configuration where the tri-subpixels are placed in "series" or coaxially. In order to compensate for the color dependence of the birefringent effect, a compensation panel or a retardation film is needed in the tri-pixel color mosaic arrangement so that it can display black and white color. When the double layer supertwisted display (DSTN) configuration is used, one of the cells is passive and usually has no electrodes. LCD with the mosaic color filter tri-pixel configuration usually has a relatively low light transmissivity as compared to the stacked cell configuration.